In many instances it is desirable to provide light reflective surfaces on various items for safety or decorative purposes, and this is often accomplished by the application of reflective coatings or elements having specular surfaces, e.g., polished metals or the like. It is also common to use sheetlike structures which have upon one or both surfaces any of various formations that are capable of reflecting impinging light rays, and this may be due principally to the angular relationship between the rays and the reflecting surface, or it may be due to the presence of a reflective coating on surfaces of the formations.
There has been a constant demand for retroreflective materials, i.e., materials capable of reflecting the predominate portion of light rays impinging thereon in a substantially parallel path toward the source of the light. Minnesota Mining and Manufacturing Company has sold a material under the trademark SCOTCHLITE which relies upon minute glass spheres embedded in a matrix of synthetic resin to provide such retroreflection. Another type of retroreflective element has employed molded members of cube corner configuration on one surface thereof, such members being of glass or synthetic plastic. Indicative of the efforts to use cube corner formations for retroreflective structures are Straubel U.S. Pat. No. 835,648 granted Nov. 13, 1906; Hedgewick et al. U.S. Pat. No. 3,258,840 granted July 5, 1966; and Jungerson U.S. Pat. No. 2,310,790 granted Feb. 9, 1943 and No. 2,444,533 granted July 6, 1948.
Cube corner reflectors molded from glass, and more recently from acrylic resins, have commonly been employed as safety devices on bicycles, automobiles and other vehicles. Although it has been suggested that the cube corner might be of small dimension, generally such formations have been relatively large in size, and the nature of the material from which the reflector has been fabricated generally resulted in structures of relatively rigid character which were not suited either for shaping for application to various substrates of nonplanar character or to use as a fabric which might be worn. Moreover, as the size of the cube corner formations is reduced criticality in control of angles and dimensions becomes far more acute, since even a minute deviation will seriously impair the ability of the material to retroflect light rays impinging thereon.
Novel composite retroreflective materials having minute, closely spaced cube corner formations, which return the great preponderance of light rays entering the front surface thereof, have recently been developed. These materials may be relatively flexible so as to permit shaping to conform to support surfaces of various configurations and to permit utilization as a fabric for application to, or formation into, wearing apparel, and they may be provided with an adhesive coating for convenient adherence to a support surface. It has also recently been proposed to provide a novel method for manufacturing such retroreflective material from synthetic resins in a manner that permits selection of resins to produce optimum characteristics; the method proposed is relatively simple and economical, and affords a high degree of control to ensure optimum development of the cube corner formations. These recent developments are the subject of copending applications for United States Letters Patent that have been filed in the names of the same inventor and assignee, now as U.S. Letters Pat. No. 3,684,348 granted Aug. 15, 1972 and U.S. Letters Pat. No. 3,689,346 granted Sept. 5, 1972.
Typically, some of the resins which may be used for the cube corner formations in such sheeting also exhibit relatively high levels of shrinkage upon transition from the liquid to the solid state. If not accounted for, such shrinkage tends to reduce very slightly the smoothness of the surface of the sheeting, which in turn produces a degree of light scattering. Use of resins exhibiting shrinkage to provide the cube corner formations is desirable in some instances and in other instances it is desirable to employ resins for the body of the cube corner formations which do not bond well directly to the sheet material by their own substance. Therefore, efforts have continued to adapt the processes of the aforementioned applications to the use of such materials.
Accordingly, it is an object of the present invention to provide novel retroreflective sheeting of substantially uniform dimensions wherein a multiplicity of minute cube corner formations provide a high level of retroflectivity and are provided by a composite structure.
It is also an object to provide such sheeting in which the cube corner formations are substantially perfectly formed and in which undesired light refraction in the sheeting retroreflectivity. minimized.
Still another object is to provide such sheeting having cube corner formations produced from a relatively shrinkable resin, which has a smooth, level front surface and which affords maximum retroreflectivity.
A further object is to provide novel apparatus for the production of high quality and uniform retroreflective sheeting, on a continuous basis.